1. Field of the Invention
The present invention relates to articles to be marked by laser marking and method for laser marking.
2. Prior Art
In the recent years, laser marking method is employed in various fields including electronic parts such as IC, resistors, condensers, inductors and the like, electric parts such as relays, switches, connectors, printed circuit boards and the like, housings of electric appliances, automobile parts, mechanical parts, cables, sheets, packaging sheets, cards, containers of foodstuffs and medical drugs, caps and labels of containers, etc. in order to mark letters or symbols denoting the name of maker, the name of article, the date of manufacture, the lot number, etc. on the surface of the articles on real time, because laser marking method enables a high-speed fine marking. As the procedure of laser marking, there is known a method of irradiating a laser onto the coating film formed on a substrate surface, eliminating the coating film of the irradiate region, and thereby forming a contrast between the laser-irradiated region and laser-unirradiated region of the substrate.
The prior method, however, is disadvantageous in that, if a high energy laser (for example, a laser of 3 J/cm.sup.2) is irradiated with the aim of forming a vivid mark, a long period of time is necessary for attaining such a high energy and the equipment therefor is expensive. Further, in case of low-strength articles such as paper, the substrate is destroyed in the laser-irradiated region and thereby the commercial value is deteriorated. The pulse type lasers is disadvantageous in that the irradiated area becomes smaller, as it is necessary to enhance an energy density in the irradiated region, owing to the low output. Although such a problem may be overcome by using a low-energy laser (for example, a laser of 1.5 J/cm.sup.2 or less), vividness of the mark is lost under such a condition. The object of the present invention consists in developing a laser marking article on which a vivid white-colored mark can be formed even in case of low-energy laser irradiation or even in case of high-speed marking.